[ARM]: Assign cost of scaling used in addressing mode for ARM cores

This patch assigns cost of the scaling used in addressing.
On many ARM cores, a negated register offset takes longer than a
non-negated register offset, in a register-offset addressing mode.

For instance:

LDR R0, [R1, R2 LSL #2]
LDR R0, [R1, -R2 LSL #2]

Above, (1) takes less cycles than (2).

By assigning appropriate scaling factor cost, we enable the LLVM
to make the right trade-offs in the optimization and code-selection phase.

Differential Revision: http://reviews.llvm.org/D24857

Reviewers: jmolloy, rengolin
llvm-svn: 284127

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 7623841a8e3..a41c4fcb9cd 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -11612,6 +11612,17 @@ bool ARMTargetLowering::allowTruncateForTailCall(Type *Ty1, Type *Ty2) const {
   return true;
 }
 
+int ARMTargetLowering::getScalingFactorCost(const DataLayout &DL,
+                                                const AddrMode &AM, Type *Ty,
+                                                unsigned AS) const {
+  if (isLegalAddressingMode(DL, AM, Ty, AS)) {
+    if (Subtarget->hasFPAO())
+      return AM.Scale < 0 ? 1 : 0; // positive offsets execute faster
+    return 0;
+  }
+  return -1;
+}
+
 
 static bool isLegalT1AddressImmediate(int64_t V, EVT VT) {
   if (V < 0)